This invention relates to a process for achieving enhanced selectivities in liquid phase separations using ion-exchange membranes exchanged with suitable olefin facilitators which exhibit unexpectedly large selectivities for the separation of multi-component mixtures of similar unsaturated hydrocarbons. Specifically, this invention relates to perfluorosulfonate ionomer membranes cast from solution and ion-exchanged with silver.sup.+ ion and exhibiting enhanced selectivities for the separation of aromatic/aromatic, alkene/aromatic, and alkene/diene mixtures is disclosed.
Due to their extensive use in the polymer industry and as solvents, there is a continuing need for better separating processes for alkenes and other unsaturated olefinic or aromatic organic compounds. Recent studies by the U.S. Department of Energy and others indicate that membrane-based separation processes for a variety of compounds are desirable, provided that membrane performance can be improved. One method for improving membrane performance is via carrier-mediated or facilitated transport. In membrane separation processes using facilitated transport, a carrier provides an additional pathway for a permeate to cross the membrane, thereby increasing the fluxes and separation factors relative to the permeates that are not facilitated by the carrier.
There are several reports of facilitated transport of olefins (single component or olefin/saturate separation) using immobilized AgNO.sub.3 solutions (Hughes et al. U.S. Pat. No. 3,758,605; Hughes et al., "Recent Developments In Separation Science", Vol. 173-199 (1986); Teramoto et al., J. Chem. Eng., Japan (19) 5,419-424 (1986). Hughes, Mahoney, and Steigelmann (1986) presented the results of a bench and pilot scale study of ethylene and propylene transport using aqueous Ag.sup.+ solutions immobilized in asymmetric, porous hollow fiber reverse osmosis membranes at ambient temperature. The ethylene/ethane separation factor for a 2M AgNO.sub.3 immobilized liquid membrane (ILM) was 243 at an ethylene partial pressure of 95 cm Hg. Teramoto et al. (1986) also studied ethylene transport with immobilized AgNO.sup.+ aqueous solutions at ambient temperature. They found selectivity for ethylene over ethane of approximately 1000 when the AgNO.sub.3 concentration was 4M and the ethylene partial pressure was 37 cmHg.
Solvent swollen ionomer films have been used as supports for ionic complexation agents in order to improve the ability of facilitated transport membranes for gas and liquid phase olefin transport (LeBlanc et al., J. Mem. Sci., Vol. 6, 339-343 (1980) Kraus, U.S. Pat. No. 4,614,524; Koval et al., I & EC Res., 28, 1020-1024, (1989); Koval et al., J.A.C.S., 110, 293-295(1988). Use of ionomer materials has been shown to improve the stability of facilitated transport membranes because the support is non-porous and the carrier cannot be removed from the membrane except by an ion-exchange reaction.
LeBlanc et al. reported ethylene transport using a Ag.sup.+ counterion carrier in a water saturated sulfonated poly(dimethylphenylene oxide) cation exchange membrane at ambient temperature. Ethylene and ethane permeance in the above system yielded values corresponding to an ideal separation factor of 288.
Kraus (1986) prepared facilitated transport membranes using Nafion.RTM. 415 containing an Ag.sup.+ carrier. However, following ion-exchange to introduce Ag.sup.+ into the membrane, the membrane was dried and soaked in an organic alcohol such as glycerol or octanol. At ambient temperature and pressure, the ethylene/ethane separation factor was 10.
The foregoing demonstrate that liquid membranes and other ion-exchange membranes containing Ag.sup.+ exhibit large (&gt;100) separation factors (flux of A/flux of B, corrected for driving force) for alkenes with respect to the alkanes due to reversible alkene-Ag.sup.+ complexation reactions. The transport of the alkene is facilitated by the Ag.sup.+, while the transport of the alkane is due to other factors, such as Fickian diffusion.
It has remained, however, for the subject invention to provide a method whereby similar molecules, such as aromatic/aromatic, aromatic/alkene, or alkene/diene mixtures can be separated in the liquid phase while achieving unexpectedly high selectivities. Such a process has proven elusive due to the affinity of suitable olefin facilitators for each component of the feed solution, resulting in a phenomenon commonly referred to as competitive transport. Since the equilibrium constants describing the complexation of facilitators with many alkenes and aromatics in aqueous solution are similar, large separation factorsdue to facilitated transport would not be anticipated.
It is an object of the present invention, therefore, to provide a process whereby similar unsaturated hydrocarbons can be effectively separated in the liquid phase using facilitated ion-exchange membranes.